Very low serotonin levels in urine testing

Just had a 24hr urine chemistry and catecholamin test done....results showed almost untraceable amount of serotonin and very low levels of epinephrine. Concerned about low levels of serotonin and wondering if levels are low in body does that mean they are low in brain?? I do take Celexa 20mg daily and being a serotonin reuptake inhibitor....what if there isn't much serotonin to reuptake..what happens then? Just wondered if I should be overly concerned. Is there a way to increase amount of serotonin in the body? Does the amount of serotonin in the body have anything to do with the amount of serotonin in the brain?Thanks